I'm looking to use "?" as my help button icon in a desktop application but I'm unsure how universally recognisable this will be.
I can localise this for different culture if necessary.
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You're wondering whether the question-mark icon is universally recognised. There are two parts to this:
I can only answer the first question. I did a quick search of the various style guides. To sum up:
Many people worldwide have been exposed to Microsoft products (edit) and Apple iOS products for years. Then again, many users are first exposed to technology via their smart phone where, despite the standard Question mark, the Help itself is less commonly provided.
As is often the case, the answer to your question is "it depends" because you don't identify the type of user, their prior experience, the type of device, and so on.
I hope these thoughts help you make a decision, or help you to focus your research.
You can do some quick research, for example by posting an image of a series of icon choices in Chalkmark, and asking people who represent your users to identify which icon signifies Help to them. Research is the best way to answer this kind of question, because when it comes to predicting usability, opinions—even of experts—are only correct part of the time.